1. Field of Invention
The present invention relates to the fields of ballistic armor for lightweight structures including aerospace vehicles and for sandwich panel construction techniques. More specifically, the present invention relates to methods and structures for sandwich panel construction that provide improved resistance to ballistic penetration.
2. Description of the Related Art
Sandwich panels have been used widely in all types of situations where light weight and high structural strength are required. Sandwich panels have been used extensively in the aerospace industry. For example, sandwich panels are used in aircraft structural parts and control surfaces as well as cabin floors and other interior panels. Sandwich panels could be, but are not yet generally used in many other structural applications where the ability to absorb relatively large amounts of impact energy is an additional important consideration. For example, U.S. Patent Publication 20050025929 discloses sandwich panels that also used in a variety of civil engineering and building construction applications where preformed structural panels are assembled together to form various exterior and interior structural elements of a building.
Sandwich panel construction is widely used in aerospace industry due to its distinct advantages in terms of stiffness, strength, stability, corrosion resistance, and above all, weight savings. One advantage of sandwich construction is that specific flexural stiffness and strength can be high. However, a notable weakness of the existing sandwich panels is that out-of-plane impact strength is very poor and can barely slow a ballistic projectile. Several approaches have been attempted to overcome the weakness of existing panels including addition of a hard ceramic front surface, often alumina or boron carbide composites, that can spread impact forces while blunting or breaking up the projectile, use of compliant or weakened sub-structures and/or a ballistic fabric anti-spall liner that can be fastened behind the panel to capture penetrating fragments or spalled chips, each with varying degrees of success.
Sandwich panels typically include two sheets of material, referred to as face sheets or rigid sheets or skins, that are bonded to opposite sides of a core to form a sandwich. The core is usually made from materials that are much more lightweight than the skins. For example, the core is most commonly a honeycomb or other foam-like cellular solid or structures. U.S. Pat. No. 5,102,723 describes a sandwich panel that uses rigid pins between the face sheets for the express purpose of mechanically stiffening and strengthening the core material. This patent also describes a sandwich panel where the face sheets and foam core form a preliminary structure and where a plurality of Z-pins is driven through one or both face sheets for reinforcement. U.S. Pat. No. 6,027,798 also discloses a pin reinforced sandwich structure that allows for a variety of combinations of the three main components, i.e., face sheets, foam core and Z-pins.
The materials used in high performance aircraft require unique blends of light weight, stiffness, strength, durability, and impact tolerance. Severe loads and environments often dictate the use of sandwich structures with minimal resistance to ballistic impact. When such panels comprise most of an aircraft's structure, vehicle survivability depends more on the location than the intensity of a weapon strike. The decision to armor has to balance protection of crew and flight critical equipment against the negative effect of extra weight on payload, mobility, and range.
Where stressed skin aircraft structures are vulnerable to high speed projectiles such as the components of disintegrating turbine engines, various military projectiles, or other high-velocity fragment sources, it is desirable that these structures or panels offer substantial resistance to penetration while simultaneously bearing the structural loads imposed on the aircraft. To date, this has generally been achieved by proving structural support and impact resistance via separate elements that are designed and fabricated independently. For this reason, existing sandwich panel technology can not achieve these two separate functions in a weight optimal fashion.
Thus, there is a recognized need in the art to provide improved sandwich panel materials and fabrication methods. Specifically, the prior art is deficient in sandwich panels that would absorb the impact energy of projectiles while minimally compromising payload capacity. The present invention fulfills this longstanding need in the art.